Case of the Month: A Backing Accident

It was summer camp time. Moms were dropping off their kids at about 9 am. Unfortunately, two of the drivers backed into each other. There were two different versions of the collision, plus the story of an unrelated witness. GEI was assigned to evaluate which version the evidence supported and to address the likelihood of injury in this incident.

The vehicles involved were a brown Ford Flex and a silver Chrysler Sebring.

The photographs and the repair estimate for the Chrysler showed the rear right corner was damaged.

The strike was on the rear face of the bumper but the energy absorption element was not damaged. The impact was delivered to the very corner of the bumper, a place that has no underlying support. This portion is similar to an empty plastic box, as it offers no meaningful resistance to impact.

The Ford was damaged differently. The rear bumper face had evidence of a strike on the rear bumper face just left of center. The bumper had only cosmetic damage. The rear license plate and frame were not bent, suggesting an impact in the one to two mph range.

So who hit whom?

The Ford driver parked nose-in on a driveway near the camp area to offload. The Chrysler then pulled up parallel to the curb into a parking space that was on the same side of the street, past or after the driveway where the Ford was parked. The witness stated that the Chrysler was parked with the rear of the vehicle hanging out into the driveway. There was an allegation that the Chrysler backed into the Ford as the Ford was backing out of the driveway, but the independent witness did not support that assertion. He said the Ford backed into the parked Chrysler, striking the part that was sticking out into the path of the driveway. After the impact, the drivers got out and recognized the other driver as another parent at the camp. They exchanged information and left. The police were not called or involved.

A scene investigation confirmed that both parties would have had a clear view of each other’s vehicles, and that there were signs prohibiting street parking. The witness also stated that the Ford driver was talking on her cell phone.

What about injuries?

The Chrysler’s bumper is rated at 2.5 mph, but when a bumper is loaded on only one side, the bumper will accept only 1.5 mph worth of impact energy without damage. The damage pattern to the Chrysler suggested an impact in the one to two mph range.

Recall that the impact was from right-to-left, and had no forward component. This was not a whiplash incident as normally considered.

The applied force to the Chrysler was about .9 g. The forces experienced by the Chrysler were so low that the occupant g-forces in this event were less than those required for injury. The occupant received forces that were, at most, about 2.7 g.

How do these data compare with published threshold injury data? Research has determined that in a retroflex impact, an occupant needs a speed change of over 7 mph coupled with a force of over 14 g to produce complaints of pain to healthy test subjects. To give the reader a feel for what forces are involved, we can compare this event with those forces experienced by riders in amusement park bumper cars. Speed changes of about 4.7 mph, and g-forces in all directions of about 2.13 g are the norm.

The conclusion was that the speeds involved were far under the published standards of threshold for injury.

In addition to being parked in a no parking zone, the Chrysler’s partial blocking of the driveway was in violation of District of Columbia Regulation §18-2405.2(a), which states one cannot park within 5 feet of a public or private driveway.

So in this collision, neither party was blameless. The Chrysler driver illegally parked, partially blocking a driveway. The Ford driver, talking on her cell phone, failed to look behind her, and backed into the stationary Chrysler.